What is Partial pressure, solubility?

In a mixture of gases each gas exerts a partial pressure proportional to its fraction: in air at $101\,\mathrm{kPa}$, oxygen ($20.9\,\%$) has $P_{\mathrm{O_2}} = 21.2\,\mathrm{kPa}$, carbon dioxide ($0.04\,\%$) $0.04\,\mathrm{kPa}$. A gas dissolves in a liquid in proportion to its partial pressure (Henry’s law): water in equilibrium with air at $15\,{}^{\circ}\mathrm{C}$ holds $0.32\,\mathrm{mmol}/\mathrm{L}$ of oxygen ($10\,\mathrm{mg}/\mathrm{L}$, $7\,\mathrm{mL}/\mathrm{L}$ of gas), against $8.7\,\mathrm{mmol}/\mathrm{L}$ ($210\,\mathrm{mL}/\mathrm{L}$) in the air itself. The partial pressure — the same in the water and the air at equilibrium — is what drives diffusion between media; the content is what a breather can extract. Carbon dioxide is twenty-five times more soluble than oxygen, so it is never the harder gas to exchange.